The long-term objective of this proposal is to understand the molecular events that lead to fate commitment and differentiation of different cell types during mammalian retinogenesis. The amacrine and horizontal cells are two classes of important interneurons that modulate and integrate visual signals in the retinal circuitry. Despite their physiological significance, however, the molecular bases underlying their development remain poorly understood at present. In this application, experiments are proposed that will focus on Foxn4, a forkhead/winged helix transcription factor that is expressed in a subset of dividing retinal progenitor cells characteristic of the subset with a fate biased toward amacrine and horizontal cells. Based on our preliminary studies it has been speculated that Fonx4 may play a key role in competence acquisition and fate commitment of these two cell types. The studies outlined in this proposal are designed to provide an integrated approach toward a comprehensive understanding of the biological function of Foxn4 during retinogenesis. Four specific aims will be pursued: i) to investigate the role of Foxn4 during retinal development by overexpression in progenitor cells. These studies aim to explore the function of Foxn4 during mammalian retinogenesis using a gain-of-function approach involving retrovirus-mediated overexpression of Foxn4 in the mouse retina; ii) to study the biological function of Foxn4 during retinal development by targeted gene disruption. The proposed experiments aim to uncover the role of Foxn4 during retinogenesis using a loss-of-function approach involving the production and characterization of Foxn4 knockout mice; iii) to analyze the relationship between Foxn4 and other retinogenic factors mediating amacrine and horizontal cell development. These studies aim to test the hypothesis that Foxn4 alone or in combination with a redundant factor, may control the genesis of amacrine and horizontal cells by activating the expression of retinogenic factors involved in the specification of these two cell types; and iv) to map the fate of Foxn4-expressing retinal progenitors. The goal is to use the Cre-loxP fate-mapping strategy to test the hypothesis that the subset of Foxn4-expressing retinal progenitors may represent those with a fate biased toward amacrine and horizontal cells. Taken together, these proposed studies are expected to provide important novel insights into the genetic regulatory networks that govern the development of different retinal cell types and may provide the foundation for better understanding and treatment of neuroretinal diseases.